Elevated interleukin-6 (IL-6) is associated with fat wasting in systemic infection and cancer cachexia. However, important new data indicates that circulating IL-6 is also elevated in human obesity, and adipose tissue is the principal source of this elevation. Because obesity is among the most prevalent public health challenges in the United States, it is important to understand IL-6 expression and function in adipose tissue, and to determine whether dysregulated expression of adipose tissue IL-6 is involved in the development of obesity and obesity-related co-morbidities. IL-6 traditionally signals through STAT3 activation, but in differentiating preadipocytes, IL-6 inhibits and STAT3 appears to be necessary for adipocyte formation. In this proposal we propose the following Specific Aims.. Aim 1: To determine mechanisms involved in IL-6-induced inhibition of adipogenesis. We hypothesize that greater than or equal to 10 ng/ml IL-6 inhibits the preadipocyte to adipocyte conversion by altering normal cell cycle progression and subsequent adipogenic transcription factor expression. We will determine in 3T3-L1 preadipocytes the effect of IL-6 on cell cycle progression and differentiation. Using chimeric GM-CSF- gp130 receptor transfections, the signaling mechanisms involved to transduce IL-6 effects will be explored. To explore the physiological relevance of IL-6 in adipose tissue, we will determine whether IL-6 knockout mice have altered adipose tissue growth, glucose, and lipid metabolism on normal and high fat diets. Aim 2: To characterize adipose tissue IL-6 expression in animal models of obesity. We find IL-6 mRNA is expressed in adipose tissue of lean mice, but it has been reported that obese db/db mice do not express IL-6 mRNA in adipose tissue. Since glucocorticoids repress IL-6 expression, we hypothesize that IL-6 expression is depressed in adipose tissue of ob/ob and db/db mice by high systemic corticosterone levels that are characteristic of these models of obesity. We also predict that IL-6 expression is elevated in adipose tissue of diet-induced obese (DIO) mice, similarly to obese humans. DIO mice retain normal corticosterone levels with the development of obesity. We will characterize IL- 6 expression in adipose tissue of lean control, DIO, ob/ob, and db/db all on a C57BL/6J background at baseline and in response to glucocorticoid agonists and antagonists, and thiazolidinediones. Aim 3: To identify the differentiation-induced STAT3 activating ligand. Our new pilot data indicate that IL-6 is not the autocrine factor responsible for MDI-induced STAT3 activation, but that the heparin binding ligand midkine (MK) is. In this Aim, we will confirm our preliminary studies, and determine whether midkine is the sole STAT3 activating ligand released upon MDI stimulation of 3T3-LI cells. Aim 4: To determine the role of STAT3 activation in adipogenesis. We hypothesize that activation of STAT3 is necessary for adipogenesis. We will block MDI-induced STAT3 activation by overexpression of PIAS3. Alternately, we will mimic STAT3 activation by overexpression of a constitutively active STAT3 to determine whether STAT3 activation alone is necessary and sufficient to induce adipogenesis. The role of STAT3 in fat pad formation will be investigated by implantation into the subcutaneous space of nude mice human preadipocytes that express vector, PIAS3, or constitutively active STAT3.